The present invention concerns a process for the continuous regulation of the power with which pastes which are intended for the fabrication of carbonaceous agglomerates are mixed.
Carbonaceous agglomerates are obtained by firing pieces of a carbonaceous paste placed in a mold. The paste is made by mixing an organic and a carbonaceous product in measured particles. The nature of the binder (coal pitch, petroleum pitch, liquid or solid pitch) and that of the carbonaceous particles (coal coke, petroleum coke, anthracite, etc . . . ) can vary a great deal. Depending on the intended use of the agglomerates, there is a stage of a prolonged mixing of the binder and the carbonaceous particles (of which the granulometric distribution is carefully controlled), at such a temperature that the binder is sufficiently fluid (60.degree. to 180.degree. C. for example) and for a length of time which assures as perfect as possible impregnation of the carbonaceous particles by the binder. The quality of the electrodes (evaluated particularly by the geometric density, the electric resistivity and the crushing resistance) produced after firing is quite dependent on the mixing efficiency.
In modern shops for the production of carbonaceous pastes-particularly for the manufacture of anodes for the production of aluminum by the Hall-Heroult process of alumina electrolysis in cryolite-the mixing of the mixture of binder plus carbonaceous particles is effected in a continuous mixing sequence which comprises one or sometimes two mixers in series.
One currently used type of mixer is shown in FIG. 1. It includes a tubular member 1 with fixed teeth 2 which are slanted in relation to the axis 3 of the tube within which a shaft 4 is moved in a back and forth movement synchronized with a rotary movement, and the shaft is provided with teeth 5 which cooperate with the fixed teeth to assure the mixing and flow of the carbonaceous paste. The fixed teeth are arranged in a helical line, and the amplitude of forward-back movement of the shaft is adjusted to the pitch of the arrangement of the fixed teeth. The discharge from the mixer(s) is through an aperture 6 which is blocked by motorized flaps 7. The opening and closing of these flaps can be controlled as a function of the thresholds of power at any given instant so as to assure satisfactory mixing of the paste and to avoid "clogging" of the apparatus, in other words its blocking with charge, as a result of a backup of excess filling.
This type of mixer has been described particularly in Swiss patents A-515 061, CH 606 498 and French Patent A-2 038 173, in the name of BUSS A. G.
The rate of opening of the discharge flaps can be manually controlled but most often regulation based on the value of the mean power consumed by the motor over a short period of time (regulation of the PID type-Proportional Integral Derivative) is used.
By observation of the current consumption curve as a function of time (directly proportional to the power in this case, with the mixer being powered with direct current), it is clear that it presents the shape of a pseudo-sinusoid of which the amplitude varies as a function of the different parameters (position of the flaps, rate of mixer filling, characteristics of the paste, etc . . . ).
The time period for this pseudo-sinusoid is equal to the time of the back and forth movement of the axis of the mixer, which is on the order of one second or slightly more.
Because of this double mechanical movement, and in the case of the use of PID-type regulation, a filter of constant RC time must be introduced in order to survey the oscillations over a short period due to the cycle of the mixer (advance and return of the main shaft).
The position of the flaps is then under control of a mean value of current which is dependent upon the time constant of the RC circuit. Preferably, the time constant T=RC is selected to be at least equal to the period of the forward-back movement of the mixer shaft. However, this simple regulation presents the drawback in some cases of not being sufficiently rapid to avoid the effects of clogging up the apparatus, particularly when one tries to use a high mixing power, near the maximum which the motor can furnish. Moreover, the user, as a security measure, uses the mixer below its maximum capacity so that a sufficient motor reserve power is available in case of clogging, to overcome the clogging and return the mixer to operation.
For a certain number of fabrications which require great regularity and the precise characteristics of the carbonaceous agglomerates, for example, anodes for electrolytic aluminum production, the process of regulation of the mixing does not always allow for the optimum quality and uniformity of the anodes which is claimed by promotors of the process, and underuse of the available capacity is quite a problem.
Additionally, in order to obtain the best quality anodes, it is necessary to optimize and/or maximize the mixing power in kilowatt-hours per ton of paste, and to apply this power very homogeneously to all of the paste being discharged from the mixer of the mixing sequence.
A very fine regulation of the mixing is thus required, meaning regulation of the rate of opening the discharge flaps of the mixer as a function of the power used at a given instant by the motor. The analog regulation only very imperfectly regulates this, since it integrates the variations of intensity of one or more cycles of rotation of the mixer shaft.